Such a sensor device has been disclosed by German Patent 3,810,008, FIG. 5.
In said patent, the carrier is designed in the form of a cylinder at one end of which there is a concavity with a considerable depth from which a spiral wire as the attachment device projects a relatively great distance. The carrier is made of a practically inflexible material. The light emitter and receiver are arranged opposite each other close to the edge of the concavity.
Fetal tissue is drawn into the concavity with the spiral so that light can be passed through the fetal tissue that is drawn into the concavity.
Within the context of this invention, it has been found that when tissue is drawn into the concavity, the arterial blood flow in this area can be impaired to the extent that no analyzable signals can be received.
With another known sensor device (European Patent 135,840, FIGS. 10 and 11), the carrier is designed as a hollow suction cup of silicone rubber with a vacuum hose leading into the concavity thereof from one side, while the opposite wall is equipped with slits arranged in a star pattern to allow the passage of air and also has a light emitter and a receiver. It is difficult to secure the suction cup reliably by means of a vacuum during the labor and delivery process, and the relatively stiff vacuum hose also causes problems.
It should be noted here that this cannot be accomplished with a weak vacuum in any case. However, if a much higher vacuum is applied, the arterial blood flow at the point of measurement in the fetal tissue is interrupted by the high pressure applied by the vacuum. Therefore, attaching the carrier by means of a vacuum of the type known previously is not very promising.